|«Introduction||Step 1: Heart Rate»|
|Author(s)||I.A.C. van der Bilt, MD|
|Moderator||I.A.C. van der Bilt, MD|
|some notes about authorship|
- 1 How do I begin to read an ECG?
- 2 What does the ECG register?
- 3 The ECG represents the sum of the action potentials of millions of cardiomyocytes
- 4 The electric discharge of the heart
- 5 The different ECG waves
- 6 The history of the ECG
- 7 The ECG electrodes
- 8 Special Leads
- 9 Technical Problems
- 10 References
How do I begin to read an ECG?
Click on the ECG to see an enlargement. Where do you start when interpreting an ECG?
- On the top left are the patient's information, name, sex and date of birth
- At the right of that are below each other the Frequency, the conduction times (PQ,QRS,QT/QTc), and the heart axis (P-top axis, QRS axis and T-top axis)
- Farther to the right is the interpretation of the ECG written (this may be missing in a 'fresh' ECG, but later the interpretation of the cardiologist or computer will be added)
- down left is the 'paper speed' (25 mm/s on the horizontal axis), the sensitivity (10mm/mV) and the filter's frequency (40Hz, filters noise from eg. lights)
- There is a calibration. At the beginning of every lead is a vertical block that shows with what amplitude a 1 mV signal is drawn. So the height and depth of these signals are a measurement for the voltage. If this is not the set at 10 mm, there is something wrong with the machine setting.
- further we have the ECG leads themselves of course, these will be discussed below.
Note that the layout is different for every machine, but most machines will show the information above somewhere.
What does the ECG register?
- The electrocardiogram
- An electrocardiogram (ECG or EKG) is a registration of the heart's electric activity.
Just like skeletal muscles, the heart is electrically stimulated to contract. This stimulation is also called activation or excitation. Cardiac muscles are electrically charged at rest. The inside of the cell is negatively charged relative to the outside (resting potential). If the cardiac muscle cells are electrically stimulated they depolarize (the resting potential changes from negative to positive) and contract. The electrical activity of a single cell can be registered as the action potential. As the impulse spreads through the heart, the electric field changes continually in size and direction. The ECG is a graphical visualisation of these electric signals in the heart.
The ECG represents the sum of the action potentials of millions of cardiomyocytes
The individual action potentials of the individual cardiomyocytes are averaged. The final result which is shown on the ECG is actually the average of billions of microscopic electronical signals. During the depolarization sodium-ions stream inwards the cell. Subsequently the calcium-ions stream into the cell. These calcium-ions give the actual muscular contraction. Finally the potassium-ions stream out of the cell. During the repolarisation the ion concentration is corrected. On the ECG, an action potential wave coming towards the electrode is shown as a positive (upwards) signal. Here the ECG electrode is represented as an eye.
|This movie shows the contraction of a single (rabbit) heart cell. The glass electrode measures the electrical current in the heart cell (with thepatch-clamp method). The electrical signal is written in blue and shows the actionpotential. Courtesy of Arie Verkerk and Antoni van Ginneken, AMC, Amsterdam, The Netherlands.|
The electric discharge of the heart
The sinoatrial node (SA node) contains the fastest physiological pacemaker cells of the heart, therefore they determine the heart rate.
First the atria depolarise and contract, after that the ventricles
The electrical signal between the atria and the ventricles goes from the sinus node via the atria to the AV-node (atrioventricular transition) to the His bundle and subsequently to the right and left bundle branch, which end in a dense network of Purkinje fibers.
The depolarization of the heart results in an electrical force which has a direction and magnitude: an electrical vector. This vector changes every millisecond of the depolarization. In the animation vectors for atrial depolarization, ventricular depolarization and ventricular repolarization are shown.
The different ECG waves
The P wave is the result of the atrial depolarization. This depolarization starts in the SA (sinoatrial) node. The signal produced by pacemaker cells in the SA node is conducted to the right and left atria. Normal atrial repolarisation is not visible on the ECG (but can be visible during atrial infarction and pericarditis).
The QRS complex is the average of the depolarization waves of the inner (endocardial) and outer (epicardial) cardiomyocytes. As the endocardial cardiomyocytes depolarize slightly earlier than the outer layers, a typical QRS pattern occurs (figure).
The T wave represents the repolarization of the ventricles. There is no cardiac muscle activity during the T wave.
One heart beat consists of an atrial depolarization --> atrial contraction --> p-wave, ventricular depolarization --> ventricular contraction --> ORS-complex and the resting phase (including the repolarization during the T-wave) between two heart beats.
Have a look at this excellent [animation of the heart cycle]
The origin of the U wave is unknown. This wave possibly results from "afterdepolarizations" of the ventricles.
The letters "Q", "R" and "S" are used to describe the QRS complex
- Q: the first negative deflection after the p-wave. If the first deflection is not negative, the Q is absent.
- R: the positive deflection
- S: the negative deflection after the R-wave
- Small print letters (q, r, s) are used to describe deflections of small amplitude. For example: qRS = small q, tall R, deep S.
- R`: is used to describe a second R-wave (as in a right bundle branch block)
See figure for some examples of this.
The history of the ECG
A concise history of the ECG is presented in a different chapter.
The ECG electrodes
Electrical activity going through the heart can be measured by external (skin)electrodes. The electrocardiogram (ECG) registers these activities from electrodes which have been attached onto different places on the body. In total, twelve leads are calculated using ten electrodes.
The ten electrodes are:
- The four extremity electrodes:
- LA - left arm
- RA - right arm
- N - neutral, on the right leg (= electrical earth or point zero to which the electrical current is measured)
- F - foot, on the left leg
It makes no difference whether the electrodes are attached proximal or distal on the extremities. However, it is best to be uniform in this. (eg. do not attach an electrode on the left shoulder and one on the right wrist).
- The six chest electrodes:
- V1 - placed in the 4th intercostal space, right of the sternum
- V2 - placed in the 4th intercostal space, left of the sternum
- V3 - placed between V2 and V4
- V4 - placed 5th intercostal space in the nipple line. Official recommendations are to place V4 under the breast in women.
- V5 - placed between V4 and V6
- V6 - placed in the midaxillary line on the same height as V4 (horizontal line from V4, so not necessarily in the 5th intercostal space)
With the use of these 10 electrodes, 12 leads can be derived. There are 6 extremity leads and 6 precordial leads.
The Extremity Leads
The extremity leads are:
- I from the right to the left arm
- II from the right arm to the left leg
- III from the left arm to the left leg
An easy rule to remember: lead I + lead III = lead II This is done with the use of the height or depth, independent of the wave (QRS, P of T). Example: if in lead I, the QrS complex is 3 mm in height and in lead III 9mm, the height of the QRS-complex in lead II is 12mm.
Other extremity leads are:
- AVL points to the left arm
- AVR points to the right arm
- AVF points to the feet
The capital A stands for "augmented" and V for "voltage".
(aVR + aVL + aVF = 0)
The Chest Leads
The precordial, or chest leads, (V1,V2,V3,V4,V5 and V6) 'observe' the depolarization wave in the frontal plane.
Example: V1 is close to the right ventricle and the right atrium. Signals in these areas of the heart have the largest signal in this lead. V6 is the closest to the lateral wall of the left ventricle.
In case of an inferior wall infarct, extra leads may be used:
- On a right-sided ECG, V1 and V2 remain on the same place. V3 to V6 are placed on the same place but mirrored on the chest. So V4 is in the middle of the right clavicle. The ECG should be marked as a Right-sided ECG. V4R (V4 but right sided) is a sensitive lead for diagnosing right ventricular infarctions.
- Leads V7-V8-V9 can be used to diagnose a posterior infarct. After V6, leads are placed towards the back. See the chapterIschemia for other ways of diagnosing posterior infarction.
Also read the chapter about Technical Problems. That will help you recognize electrical disturbances and lead reversals.